From “A New Method for On-line Determination of Residence Time Distributions in Extruders”, J. Golba, Polymer Physics & Engineering Branch Corporate Research and Development Center Schenectady, New York Technical Papers, Regional Technical Conference—Society of Plastics Engineers (1980) p. 83 to 87, it is known that the experimental determination of residence time distributions (RTD) is a critically important aspect of most analyses of the extrusion process. In essence, these distributions quantify the thermal and deformational histories experienced by the polymer system during processing. Such considerations are not limited to just an average set of process conditions for some average value of time. Instead the history of the polymer may be described in terms of a set of process conditions over a distribution of times. With appropriate models the variance of the RTD can be correlated with overall mixing in the extruder. The problematic tail of the distribution resulting from stagnant flow can also be characterized. In the realm of the extruder chemistry a more quantitative description of reactor behaviour is possible. Finally, RTD's play an important role in the experimental verification of theoretical models for extrusion processes. In fact, in many cases where the more common melt pressure profile-throughput experiments are not possible, e.g., starve fed co-rotating twin screw extrusion, they provide the most useful data for comparison.
The most common technique for experimentally determining RTD uses a pulse input or a tracer. The extruder is first equilibrated under a given set of operating conditions. The tracer is injected and the extrudate collected and sectioned over some specified time interval. Each section is then analysed for tracer concentration and the appropriate distribution curve is constructed. A number of different tracers can be used depending on the analytical capability available. While this procedure is quite acceptable, it tends to be very time consuming and relatively expensive. As a result extensive mappings of RTD's as a function of screw design, operating characteristics, and process material properties are nearly impossible to determine. An alternative approach would be the continuous, on-line determination of residence time distributions. Such a methodology should permit, at least in principle, the rapid generating of a large number of curves necessary to more completely characterize an extrusion process.